YZ450 too much compression

8 replies to this topic
  • chriskeit

Posted February 05, 2011 - 09:32 AM


Bike was fine 2 weeks ago when finished my last ride. Today when trying to start it, the compression stroke is very hard! Managed to bump start but it didn't have much power and wanted to die the whole time.

I removed the sparkplug and the engine turns easy, as soon as I put the spark back in it's hard to turn over again.

Any ideas?

  • grayracer513

Posted February 05, 2011 - 09:46 AM


Either the exhaust cam has jumped a tooth, or the auto decompression system is stuck. Cam timing is by far the more likely to be the problem.

If you find the cams out of time, you will probably also find the chain has a few tight links in it. Change the chain and the tensioner, and don't try running it again until you do.


  • brentn

Posted February 05, 2011 - 11:03 AM


Both bad crank bearings and a skipped tooth on the cam show these symptoms, how can you diferentiate the two by how the bike is running? I'm sure they would both run poor, but I suppose that the crank gears would probably die right away if it was an issue?

  • grayracer513

Posted February 05, 2011 - 12:31 PM


...bad crank bearings ...(will) show these symptoms, ...

Really? Explain that.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • brentn

Posted February 05, 2011 - 12:45 PM


Well when the crank bearings are shot I'm sure there are times when they are rough, making the bike overall hard to kick over and impossible to start? I swore I had read that in this forum.

  • grayracer513

Posted February 06, 2011 - 10:35 AM


Maybe, but the crank bearings are not likely to suddenly feel nice and smooth when you take the plug out.

  • brentn

Posted February 06, 2011 - 11:34 AM


Didnt read the plug part....

  • chriskeit

Posted February 07, 2011 - 01:27 AM


Thanks for the input grayracer513 and brentn. I have done the trick of rolling the engine backwards to release the Autodecompression with no luck. I am convinced that the Exhaust Cam has jumped a tooth (Speculation as I have't taken the top off yet).

Everything has been running very smooth so I wouldn't think bearing would suddenly go.
I have also read another TT forum with a very same issue and the same diagnosis.

The cam is a bit beyond my experiance so I will take the bike into mechanic to look at the cam and tentioner (and replace) along with Valve clearance.

I read that the cam chain should be replaced every 60hrs or so, I have never changed this since I have had the bike (07 model)

Once more question? Do the valves overlap with piston (YZ450F 07)... I am worried that if the cam can jump a tooth, the engine will blow!?

  • grayracer513

Posted February 07, 2011 - 07:27 AM


If you mean can the valves contact the piston if out of time? Very definitely yes, they can and they do. It all depends on which cam moves which way, and how much.

In your case, I'd say you were probably OK, as the bike still has good compression. If the valves had bent, you'd have very little, and your clearances would be looser than normal.

Related Content


Should I buy this bike? help....... by Numsa Hul

Dirt Bike   General Dirt Bike Forums   What bike should I buy?
  • * * * * *
  • 10 replies

The "going back to the other darkside" adventure... by Monk

Dirt Bike   Make / Model Specific   KTM   250/350/450/505 SX-F & XC-F (4-Strokes)
  • Hot  113 replies

Wilson the Blue Privateer by jetfuel

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  50 replies

Megabomb Fitment by 288yz450

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply

James Stewart back on a YZ450F by YamaLink

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  47 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.